Mine roof support with means for lifting the base elements

ABSTRACT

The invention provides an advancing mine roof support in which successively advanced units each comprise a base element, hydraulic props thereon and a superstructure carried by the props with the base element and superstructure of an inner one of the units disposed in longitudinally extending medial gaps afforded by the base element and superstructure of an outer one of the units, the base element being connected by a double-acting advancing ram which is connected hydraulically in a circuit providing pressurisation of the foremost double-acting prop or props of at least the leading unit so that the base element thereof is lifted during advance to ride over floor obstructions.

United States Patent [1 1 Groetschel July 31, 1973 1 MINE ROOF SUPPORT WITH MEANS FOR LIFTING THE BASE ELEMENTS [76] Inventor: Karl Maria Groetschel, Stolzestrasse [30] Foreign Application Priority Data June 16, 1971 Germany P 21 29 749.4

[56] References Cited UNITED STATES PATENTS 12/1963 Joseph 61/45 D 3,143,862 8/1964 Cowlishaw 61/45 D 3,240,022 3/1966 Mills et al. 61/45 D FOREIGN PATENTS OR APPLICATIONS 92,217 9/1968 France 61/45 D 6/1964 France .1 61/45 D 2/1960 Germany 61/45 D Primary ExaminerDennis L. Taylor Attorney-George H. Spencer, Harvey Kaye et a1.

[57] ABSTRACT The invention provides an advancing mine roof support in which successively advanced units each comprise a base element, hydraulic props thereon and a superstructure carried by the props with the base element and superstructure of an inner one of the units disposed in longitudinally extending medial gaps afforded by the base element and superstructure of an outer one of the units, the base element being connected by a doubleacting advancing ram which is connected hydraulically in a circuit providing pressurisation of the foremost double-acting prop or props of at least the leading unit so that the base element thereof is lifted during advance to ride over floor obstructions.

16 Claims, 10 Drawing Figures PAIENIEU I975 3.748.861

SHEET 2 [1F 3 MINE ROOF SUPPORT WITH MEANS FOR LIFTING THE BASE ELEMENTS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to mine roof supports, hereinafter referred to as being of the kind specified, comprising successively advanceable units each including a base elemnet having at least two hydraulically extensible props projecting upwardly therefrom at positions spaced apart longitudinally of the direction of advancement of the support, and roof-engaging superstructure carried by said props, and hydraulically operated advancement means for effecting advancement of the units in succession whilst its props are contracted to bring the roof-engaging superstructure out of loadbearing relation with the roof.

The unit which, in each cycle of advancement, will first encounter a transversely extending floor step or obstruction in the path of the support as a whole, whether by virtue of the sequence of advancement of the units or by virtue of the relative positions of the forward ends of the base elements at the beginning of the cycle, is herein called the leading unit and the other unit is called the trailing unit.

2. Description of the prior art Various constructions have been previously proposed involving the retraction of the hydraulic props of the unit undergoing advancement so that floorengaging elements at the lower ends of these props are raised clear of the floor, while th unit is in effect suspended by means of engagement of its superstructure with the superstructure of the other unit which is maintained in load-bearing relation with the roof of the mine working by pressurisation of its hydraulic props.

All these proposals have, however, substantial disadvantages.

Firstly, in certain cases of longitudinally spaced props of each unit are not connected to and carried by a base element but are provided with individual floorengaging elements, with the result that during advancement engagement of the lower ends of the props or their floor-engaging elements with any floor obstruction is liable to cause distortion of the props and jamming of the pistons thereof.

Secondly, in certain cases the manner of engagement between the superstructure of the two units has provided only inadequate stability for the unit undergoing advancement thereby increasing the likelihood that tilting ofthis unit will take place and its props will be brought into contact with floor obstructions and damaged.

Thirdly, the support as a whole has been relatively complex as regards its construction and sequence of operations to be performed, calling for a high degree of skill on the part of the operating personnel.

The main object of the present invention is to provide a new or improved form of advancing mine roof support of the kind specified which will be capable of reliable operation over irregular floors, whether these be horizontal or present an inclination, and by means of which one or more of the disadvantages of the prior proposals are overcome or reduced.

SUMMARY OF THE INVENTION According to the present invention there is provided a mine roof support comprising an outer unit including an outer base element having laterally spaced parts defining a longitudinally extending gap therebetween, hydraulic props respectively mounted on said parts adjacent their forward ends, hydraulic props respectively mounted on said parts at respective positions spaced rearwardly along said parts from their forward ends, outer roof-engaging superstructure carried by said props, projecting forwardly of said outer base element, and defining a longitudinally extending gap in the central region thereof, an inner unit including an inner base element disposed in and movable along said gap of said outer base element, longitudinally spaced hydraulic props mounted on said outer base element, inner roof-engaging superstructure carried by said props on said inner base element, projecting forwardly of said outer base element, and disposed in said gap de fined by said outer superstructure, hydraulically energisable advancing means operatively engaged between said inner and outer units for advancing each of said units when its props are contracted to relieve said superstructure carried thereby from load-bearing relation with the roof while the props of the other unit are extended to maintain said superstructure carried thereby in load-bearing relation with the roof, double-acting piston and cylinder means incorporated in at least the foremost prop or props of at least the leading one of said units, hydraulic circuit means interconnecting said double-acting prop or props and said advancing means for energising the former in a mode to lift the base element of said leading unit in response to resistence to advancement of said leading unit, and support bearing means on said superstructure of the trailing one of said units for supporting said superstructure of said leading unit when lowered during advancement thereof.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described, by way of example, with reference to the accompanying drawings wherein:

FIG. 1 is a view in front elevation of one embodiment of roof support of the kind specified in accordance with the invention;

FIG. 2 is a view in side elevation of the embodiment illustrated in FIG. 1 showing in broken lines the position of the outer unit of the support when advancing over a floor obstacle;

FIG. 3 is a fragmentary view on an enlarged scale and in cross-section on the line III-III of FIG. 1 showing the frontal portion of the forwardly projecting part of the superstructure of the inner unit and the support bearing provided thereon for the superstructure of the outer unit;

FIG. 4 is a view in side elevation of a second embodiment of roof support of the kind specified in accordance with the invention; the superstructure of the outer unit shown partly in vertical cross-section;

FIG. 5 is a fragmentary view in vertical cross-section through the forwardly projecting portion of the superstructure of the inner unit of the embodiment of FIGS. 1 and 2 showing a modified arrangement for mounting the support bearing;

FIG. 6 is a view in cross-section on the line VI-VI of FIG. 5;

FIG. 7 is a hydraulic circuit diagram in schematic form showing the connections of the advancing means and certain of the props to a pressure source of hydraulic fluid;

FIG. 8 is a fragmentary view in side elevation and partly in vertical cross-section showing the manner of connection of each of the double-acting props to the associated floor bar;

FIG. 9 is a plan view of the parts shown in FIG. 8 in cross-section on the line IX-IX of FIG. 8;

FIG. is a fragmentary view in side elevation of the superstructure of the outer unit modified to include a slide face having a steep initial portion.

DESCRIPTION OF THE PREFERRED EMBODIMENT The mine roof support shown in FIGS. 1 and 2 comprises broadly a three-frame structure in which the two outer frames afford a gap therebetween for accommodating an inner support unit, the base parts of the two outer frames and the superstructure parts of the two outer frames being joined to form a chock or trestle comprising an outer unit which is stable in itself.

The base element of this outer unit A thus comprises two laterally spaced floor-engaging bars 1 having forward prop sockets 2 and rearward prop sockets 3 fixed thereto, a cross bar 4 connecting the prop sockets 2 with each other, and a shield-like cross member 5 connecting the rear prop sockets 3 with each other. The outer unit includes two forward props 6 which are connected within the sockets 2 with the floor bars 1 so as to be capable of lifting the floor bars 1 when the lower sections of the props 6 are retracted upwardly relatively to their inner sections, the props 6 being doubleacting props incorporating pistons 60 (FIG. 8) within the lower sections providing annular chambers 6b above the pistons and cylindrical chambers 6c below the pistons. The rear props 7 may be single-acting props although double-acting props similar to the props 6 seen in FIG. 8 may be utilised if desired. The props 6 and 7 serve to support roof bars 9 which comprise laterally spaced parts of the superstructure of the outer unit and which, as illustrated, are connected by means of transverse bars 8 to form a frame.

As seen in FIGS. 8 and 9, each of the props 6 is connected to the associated floor bar by connection means allowing the prop some freedom to tilt relatively to the floor bar while exerting a lifting force thereon when the prop is contracted.

Thus the prop has a radially projecting stud 53 welded to its outer component or cylinder and which engages with clearance beneath a bolt 54 forming the top of an inverted stirrup secured to the floor bar 1.

The inner frame which constitutes the inner unit B of the support is guided for longitudinal movement with respect to the outer unit of the support by guide elements not specifically indicated, such inner unit being disposed within a gap afforded between the two outer frames of the outer unit and bounded at its ends by the cross bar 4 and cross member 5. Two longitudinally spaced props l I of the inner unit are mounted in sockets l3 fixed on a central floor bar 12 and provide support to a central roof bar 14 forming the superstructure of the inner unit. At least the foremost of the props 11 may, as seen in FIG. 7, be of the double-acting type having spaces Ilb and 1 1c above and below the piston Ila. The props I1 and intervening part of the floor bar are at all stages contained within the length of the gap afforded by the base element of the outer unit. The outer unit thus constitutes the leading element by virtuc of the face that the forward end of its base element is always situated ahead of that of the inner unit and hence will first encounter any transverse step or floor obstruction lying across the path of the support as a whole.

As will be noted from FIG. 2, the support bearing 16 is provided forwardly of a plane passing through the axes of the forward props 6 of the outer unit and forwardly also of the connecting cross member 8 of the superstructure, such bearing incorporating two arms 15 projecting laterally beyond the central roof bar 14 to extend beneath the two outer roof bars 9 of the outer unit. The support bearing 16 may incorporate rollers 17 rotatable on the arms 15. In the embodiment illustrated the rollers 17 engage only the lateral margins of the outer roof bars 9 nearest the central roof bar 11. In a construction, however, wherein the outer roof bars 9 are connected with each other in a manner permitting of some flexure or angular movement in a vertical transverse plane, the supporting arms 15 may be made longer so that they extend at least beneath the medial axis or centre line of each roof bar 9, as indicated in broken lines in FIG. 1.

In the embodiment illustrated the advancement means provided is in the form of a hydraulic ram 18 arranged within the base element of the inner unit and connected, as seen in FIG. 7, at its rearward end to the base element 1 of the inner unit and at its forward end with a downwardly extending projection I9 on the cross bar 4 of the base element of the outer unit. It will be understood that the ram 18 is a double-acting ram and can be pressurised alternately in the cylinder spaces 18a, 18b at opposite sides of its piston to ad- Vance the outer unit when the props 6 and 7 of the latter are contracted to relieve this unit of load-bearing relation with respect to the roof while the props ll of the inner unit are pressurised to maintain this unit in load-bearing relation to the roof, or to advance the inner unit within the gap afforded by the outer unit when the props of the inner unit are contracted and those of the outer unit are extended to establish this in load-bearing relation to the roof.

In the former case, that is when the inner unit B is in load-bearing relation to the roof and the outer unit A is advancing, the props 6 act as a part of a lifting means in consequence of pressurisation of the annular chambers 6b in the interior of the lower sections of the props 6 above the pistons 6a thereof. A further part of the lifting means comprises a contact face consisting of slide faces 20 at the underside of the forward regions of the outer roof bars 9which engage the supporting arms 15 through the intermediary of the rollers 17, such supporting arms being secured to the central roof bar 14. In the embodiment illustrated in FIGS. 1 and 2 the slide faces 20 on the roof bars 9 incline upwardly in a direction towards the forward extremity of each roof bar 9.

The double-acting props 6 of the outer unit A, and preferably also the foremost prop ll of the inner unit B when such prop is of the double-acting type, are connected, as shown in FIG. 7, in a hydraulic circuit with a source of hydraulic fluid under pressure such as a pump 39, and a manually settable distributor valve 40. The advancing double-acting ram 18 of the circuit also containing a throttle valve 49 which is adjustable manually as indicated diagrammatically by the arrow.

For simplicity connections from the pump through the valve 40 to the remaining props are omitted from the drawing. The distributing valve 40 is supplied with fluid under pressure through a pipe 41 and has outlets 42, 43, 44 and 45, of which the first two are connected by pipes 46 and 47 to cylinder spaces 11b and 11c of the prop l1, and also by pipe 48 through throttle valve and pipe 48a to the space 18b of the ram 18. The second pair of outlets 44 and 45 are connected respectively by pipes 50 and 51 to the pairs of cylinder spaces 6b, 6c of the double-acting props 6 and by pipe 52 to the space 18a of the ram 18. For effecting advancement of the inner unit B the distributing valve 40 is set to a position in which fluid under pressure is delivered from outlet 42. This pressurises the upper space 11b of the foremost prop 11 of the inner unit and causes this to contract, thereby relieving the inner unit of roof load. As soon as the roof bar 14 of the inner unit descends to an extent such that it rests on the cross members 8, further contraction of the prop 11 will tend to lift the forward end of the floor bar 12. At the same time pressure transmitted through pipes 48, valve 49 and pipe 48a to the space 18b (space 18a being connected by pipe 52 to exhaust) will cause the ram 18 to contract thereby drawing the floor bar 12 of the inner unit, and hence the whole of the inner unit, forwardly in the gap afforded by the outer unit.

If the lifting force exerted by the foremost prop is not sufficient to lift the floor bar 12 of the inner unit because of slow build up of pressure in the chamber 18b then the throttle valve 49 can be operated to increase the degree of throttling and reduce the flow of pressure fluid to the chamber 18b, thereby increasing the build up of pressure in the annular chamber 1 1b. The throttle valve thus simulates the presence of an obstruction to advancement presented by the floor and situated in the path of the floor bar 12.

When advancement is completed both props 11 of the inner unit are again set, i.e., extended, by operation of the distributing valve 40 to supply fluid under pressure from outlet 44 to space 110, space 11b at this time being connected through pipe 46 to exhaust.

For effecting advancement of the outer unit A, outlet 44 is pressurised by appropriate setting of the distributing valve 40 to cause build up of pressure in the spaces 6b of the props 6 (spaces 60 being connected through pipe 51 and outlet 45 to exhaust). Simultaneously pressure is transmitted through pipe 52 to space 180 of the advancing ram 18. The superstructure 9 of the outer unit A thus descends onto the support bearing 16 suspended from the roof bar 14 of the inner unit B and thereafter the contraction of the props 6 will tend to lift the floor bars 1 of the outer unit A. Simultaneously pressure will build up in chamber 18a. If the throttle valve 49 is fully open there will be substantially no build up of pressure in the chamber 18b connected by pipes 480, valve 49 and pipes 48 and 46 to outlet connected to exhaust. When the piston of the ram 18 moves forwardly to advance the base element of the outer unit, and hence the whole outer unit, the resistence to movement will arise only with frictional engagement with the floor or engagement with any obstructions thereon. If this resistance is not sufficient to create the necessary build up of pressure in the chambers 6b to lift the base element of the outer unit, then the exhaust of fluid from the chamber 18b can be reduced by partial closing of the throttle valve 49 (simulating the existence of an obstruction in the floor).

Instead of connecting the throttle valve 49 in series with pipes 48, 48a as shown, it may be connected in series with pipe 52 below the junction with pipe 50 or a throttle valve may be provided at both these positions.

An operation is illustrated typically in FIG. 2 wherein the advancing outer unit A is shown encountering a step or rise in the floor. The resistance to forward movement is sensed by the ram 18 and causes pressure in the spaces 6b to rise as described to effect lifting of the forward ends of the floor bars 1 which together form the base element of the outer unit.

It will of course be understood that the upper ends of the inner sections of the props 6 and foremost prop 11 are anchored to the roof bars 9 in a manner preventing downward withdrawal therefrom.

FIG. 2 further diagrammatically illustrates the dimensions and preferred disposition of the slide faces 20 which collectively constitute the contact face of the superstructure of the outer unit comprising the outer roof bars 9. a indicates the length of the slide faces 20 which are engaged by the support bearing 16 during a full forepoling step, i.e., a full stroke of the ram 18, during advancement of the outer unit A with the inner unit B in a horizontal position. This effective length a of the slide faces 20 which would be sufficient under the most favourable conditions of operation (a completely'horizontal floor) if, however, not adequate for working conditions encountered underground in which the inner unit is not horizontal in the fore and aft direction. For this reason each slide face 20 is extended at the forward and rearward limits of the length a by distances b, 0 respectively to ensure that it affords portions for continued operative engagement with the support bearing 16 in the event of relative inclination in the fore and aft direction between the inner and outer units when the latter occupy end positions at the limits of their relative travel. The extensions b and c of the slide faces require to be made longer as the construction or height of the support as a whole increases and/or as the size of the floor irregularities to be negotiated increases, because of the possible increase in relative angular displacement between the outer and inner parts of the superstructure under such conditions.

In some cases it is desirable to provide an artificial increase to advancement of the outer unit as the beginning of the travel thereof in order to establish a build up of pressure in the spaces 6b of the double-acting props 6 independently of the setting of the throttle valve or as a possible substitute for provision of the latter. Such an arrangement is illustrated in FIG. 10 wherein each slide face 20 is formed at its leading end with a portion 20' having an upward and forward inclination with respect to the horizontal which is greater than that presented by the remainder of the slide face. Engagement of the support bearing 16 by the portion 20' of the slide face thus not only increases the hydraulic pressure in the spaces 6!) but also provides, by cam action with respect to the support bearing 16, a lift of the roof bars 9 in response to forward movement of the outer unit as a whole.

The support bearing 16 is shown at 16 on a somewhat larger scale in FIG. 3 together with a forwardly projecting part of the central roof bar 14 on which it is carried. The supporting arms 15 are shown collectively at 15 and are maintained (when unloaded) in engagement with the underside of the roof bar 14 or with a fixed stop 32 provided in this position by a biasing means which may comprise a leaf spring 31 secured to the forward end of the roof bar 14 and to the arms 15.

The arms have secured thereto an upwardly extending shank or rod 33 provided at its upper end with a stop member 34 of a size greater than the opening in the roof bar 14 or plate 32 through which the shank or rod 33 passes, and thereby providing a stop to limit downward displacement of the rods 15' against the action of the spring 31 when under load from the roof bars 9. Load from the roof bars 9 is transmitted to the roof bar 14 partly through the spring 31 and, when the arms 15' have been fully lowered, through the stop member 34.

In FIG. 4 there is diagrammatically illustrated a further embodiment of roof support of the kind specified in accordance with the invention in various positions which the support assumes in the course of surmounting a steep and comparatively high floor step 22. The broken lines indicate the initial position of the outer unit of the support preparatory to climbing the floor step 22 and in which the outer roof bars 9 are removed from contact with the roof whilst the roof-engaging bar 14 of the inner unit already occupies its advanced position. The full lines show the outer support unit raised in its forward region whilst the rearward region thereof is still supported from the floor. The outer unit is thus in a position into which it has been moved with the aid of the lifting means comprising the forward props 6 and the support bearing 16.

If for raising the frontal region of the base element from the initial position shown in broken lines into the illustrated floating position, the pressure in the annular chambers above the pistons in the lower sections of the two leading props 6 is insufficient, the throttle valve 49 is adjusted to modify the ratio of lift effected by contraction of the prop 6 to advancement affected by oper ation of the ram 18 so as to increase this ratio in comparison with that which would exist were the throttle valve 49 not incorporated in the hydraulic supply circuit. After operation to effect lifting of the forward region of the outer unit, the throttle valve 49 may be restored to its initial position.

During lifting of the outer unit the latter is tilted about the floor-engaging rearward edge of the floor bars 1. As a result of this the slide faces 20 on the undersides of the outer roof bars 9 are displaced rearwardly relative to the support bearing 16 through a distance dependent upon the angle of tilt and the overall height of the roof support. In order to ensure that the outer roof bars 9 which require support from the relief bearing 16 do not fail to make contact therewith, the length of the slide faces has to be increased as already mentioned to an extent exceeding that which would be required for advancement of the support on a level floor. In the embodiment shown in FIG. 4 a modification which is made for this purpose is that extension portions of the slide faces are presented by the underside of an extension piece 24 adapted to be extracted forwardly from the associated roof bar 9. At its forward end each extension piece 24 has a downwardly extending nose or projection 25 behind which the relief hearing 16 engages during advancement of the inner unit, so that the extension piece 24 is automatically extracted to the required length from the associated outer roof bar 9. The stop member 26 in the interior of the roof bar 9 prevents complete extraction of the exten- 6 piece 24 automatically to the retracted position when it is out of load-bearing relation with the support bearing 16, due to extension of the props 6 and 7 when the outer unit is again brought into load-bearing relation with the roof.

FIGS. 5 and 6 show a modified form of support bearing 16' provided with means for height adjustment with respect to the roof bar 14. The mounting arrangement comprises an upwardly extending shank 37 secured to the arms 15 and extending through an opening in supporting plate 36 fixed to the central roof bar 14 with the shank projecting into the roof bar interior. The shank 37 has a plurality of vertically spaced holes. By inserting a wedge 38 or other retaining element, such as a bolt, through one of the holes 35, it is possible to retain the support bearing 16' relative to the central roof bar 15 at any of a plurality of vertical distances therefrom, said wedge or other retaining element engages the upper side of the plate 36 in the manner shown in FIG. 5. By removing the wedge 38 or other retaining element the support bearing 16 may be detached from the roof bar 14, for example for use with another roof support in the working. The shank 37 may be of rectangular cross-section and may pass through a rectangular hole in the piate 36 of appropriate dimensions to retain the shank against rotation about a vertical axis.

I claim:

1. A mine roof support comprising:

a. an outer unit including i. an outer base element having laterally spaced parts defining a longitudinally extending gap therebetween,

ii. hydraulic props respectively mounted on said parts'adjacent their forward ends,

iii. hydraulic props respectively mounted on said parts at respective positions spaced rearwardly along said parts from their forward ends,

iv. outer roof-engaging superstructure carried by said props, projecting forwardly of said outer base element, and defining a longitudinally extending gap in the central region thereof,

b. an inner unit including i. an inner base element disposed in and movable along said gap of said outer base element,

ii. longitudinal spaced hydraulic props mounted on said outer base element,

iii. inner roof-engaging superstructure carried by said props on said inner base element, projecting forwardly of said outer base element, and disposed in said gap defined by said outer superstructure,

c. hydraulically energisable advancing means operatively engaged between said inner and outer units for advancing each of said units when its props are contracted to relieve said superstructure carried thereby from load bearing relation with the roof while the props of the other unit are extended to maintain said superstructure carried thereby in load-bearing relation with the roof,

d. double-acting piston and cylinder means incorporated in at least the foremost prop or props of at least the leading one of said units,

e. hydraulic circuit means interconnecting said double-acting prop or props and said advancing means for energising the former in a mode to lift the base element of said leading unit in response to resistence to advancement of said leading unit,

f. and support bearing means on said superstructure of the trailing one of said units for supporting said superstructure of said leading unit when lowered during advancement thereof.

2. A mine roof support according to claim 1 wherein:

a. said superstructure of said leading unit has a downwardly presented longitudinally extending contact face for engagement with said support bearing means,

b. said contact face has a length exceeding the maximum advancement step which said advancement means is capable of imparting to said leading unit.

3. A mine roof support according to claim 1 wherein:

a. said superstructure of said leading unit has a downwardly presented longitudinally extending contact face for engagement with said support bearing means,

b. said contact face is inclined upwardly in a forward direction.

4. A mine roof support according to claim 1 wherein:

a. said superstructure of said leading unit has a downwardly presented longitudinally extending contact face for engagement with said support bearing means,

b. said contact face has a portion which inclines upwardly in a forward direction at an angle which is steeper than that presented by the remainder of said contact face to provide an increase in resistance to advancement of said leading unit and hence an increase in lift applied by said doubleacting prop or props as this portion of said contact face engages said support bearing means.

5. A mine roof support according to claim 1 further including means for adjusting the height of said support bearing means.

6. A mine roof support according to claim 1 wherein said support bearing means is carried on that part of said superstructure of said trailing unit which projects forwardly of said base element of said leading unit.

7. A mine roof support according to claim 6 wherein:

a. said part of said superstructure of said trailing unit has an opening,

b. said support bearing means has a shank projecting upwardly therefrom through said opening,

c. retaining means is provided cooperative with said shank above said opening to secure said support bearing means against descent.

8. A mine roof support according to claim 6 wherein:

a. yieldable biasing means is provided for urging said support bearing means upwardly with respect to said part of said superstructure of said trailing unit,

b. stop means is provided to permit descent of said support bearing means with respect to said part of said superstructure of said trailing unit.

9. A mine roof support according to claim I wherein said support bearing means is connected to said superstructure of said trailing unit by manually releasable means.

10. A mine roof support according to claim 1 wherein said hydraulic circuit means includes means for proportioning flow of hydraulic fluid from a-pressure source respectively to said double-acting prop or props and to said advancing means to provide an increase in the ratio of lift of said base element to said leasing unit to advancement thereof.

11. A mine roof support according to claim 1 wherein:

a. said double-acting piston and cylinder means are incorporated in at least the foremost props of said outer unit,

b. said props and the portion of said base element of said inner unit extending between these props are at all stages of advancement of both of said units contained within the length of said gap afforded by said base element of said outer unit.

12. A mine roof support according to claim 11 wherein said double-acting piston and cylinder means is incorporated also in at least the foremost prop of said inner unit.

13. In a mine roof support comprising inner and outer units each having a base element, longitudinally spaced hydraulically extensible props thereon, and roof-engaging superstructure carried by the props and projecting forwardly of the base element of the outer unit, with the base element and superstructure of the inner unit disposed in longitudinal gaps defined between laterally spaced members of the base element and superstructure of the outer unit, and the two units being relatively longitudinally movable by hydraulically energisable advancing means, the improvement comprising:

a. support bearing means on said part of the superstructure of the upper unit which projects forwardly of the base element of the outer unit,

b. a contact face on that part of the superstructure of the outer unit which projects forwardly of the base element thereof and engageable with said support bearing means upon lowering of the superstructure of said outer unit,

c. double-acting piston and cylinder means in at least the foremost props of said outer unit which latter are liftingly connected with the base element of said outer unit.

14. The improvement claimed in claim 13 wherein said support bearing means comprises:

a. shank means supported from said part of the superstructure of said inner unit and projecting downwardly therefrom,

b. laterally projecting members carried by said shank means adjacent to its lower end and underlying respective ones of said laterally spaced members of the superstructure of said outer unit.

15. The improvement claimed in claim 13 wherein said support bearing means further includes rollers on said laterally projecting members beneath respective ones of said laterally spaced members of the superstructure of said outer unit to provide rolling engagement therewith during advancement of said outer unit.

16. The improvement claimed in claim 13 wherein said support bearing means is urged upwardly with respect to said part of the superstructure of said inner unit by spring means acting between said part and said support bearing means. 

1. A mine roof support comprising: a. an outer unit including i. an outer base element having laterally spaced parts defining a longitudinally extending gap therebetween, ii. hydraulic props respectively mounted on said parts adjacent their forward ends, iii. hydraulic props respectively mounted on said parts at respective positions spaced rearwardly along said parts from their forward ends, iv. outer roof-engaging superstructure carried by said props, projecting forwardly of said outer base element, and defining a longitudinally extending gap in the central region thereof, b. an inner unit including i. an inner base element disposed in and movable along said gap of said outer base element, ii. longitudinal spaced hydraulic props mounted on said outer base element, iii. inner roof-engaging superstructure carried by said props on said inNer base element, projecting forwardly of said outer base element, and disposed in said gap defined by said outer superstructure, c. hydraulically energisable advancing means operatively engaged between said inner and outer units for advancing each of said units when its props are contracted to relieve said superstructure carried thereby from load bearing relation with the roof while the props of the other unit are extended to maintain said superstructure carried thereby in load-bearing relation with the roof, d. double-acting piston and cylinder means incorporated in at least the foremost prop or props of at least the leading one of said units, e. hydraulic circuit means interconnecting said double-acting prop or props and said advancing means for energising the former in a mode to lift the base element of said leading unit in response to resistence to advancement of said leading unit, f. and support bearing means on said superstructure of the trailing one of said units for supporting said superstructure of said leading unit when lowered during advancement thereof.
 2. A mine roof support according to claim 1 wherein: a. said superstructure of said leading unit has a downwardly presented longitudinally extending contact face for engagement with said support bearing means, b. said contact face has a length exceeding the maximum advancement step which said advancement means is capable of imparting to said leading unit.
 3. A mine roof support according to claim 1 wherein: a. said superstructure of said leading unit has a downwardly presented longitudinally extending contact face for engagement with said support bearing means, b. said contact face is inclined upwardly in a forward direction.
 4. A mine roof support according to claim 1 wherein: a. said superstructure of said leading unit has a downwardly presented longitudinally extending contact face for engagement with said support bearing means, b. said contact face has a portion which inclines upwardly in a forward direction at an angle which is steeper than that presented by the remainder of said contact face to provide an increase in resistance to advancement of said leading unit and hence an increase in lift applied by said double-acting prop or props as this portion of said contact face engages said support bearing means.
 5. A mine roof support according to claim 1 further including means for adjusting the height of said support bearing means.
 6. A mine roof support according to claim 1 wherein said support bearing means is carried on that part of said superstructure of said trailing unit which projects forwardly of said base element of said leading unit.
 7. A mine roof support according to claim 6 wherein: a. said part of said superstructure of said trailing unit has an opening, b. said support bearing means has a shank projecting upwardly therefrom through said opening, c. retaining means is provided cooperative with said shank above said opening to secure said support bearing means against descent.
 8. A mine roof support according to claim 6 wherein: a. yieldable biasing means is provided for urging said support bearing means upwardly with respect to said part of said superstructure of said trailing unit, b. stop means is provided to permit descent of said support bearing means with respect to said part of said superstructure of said trailing unit.
 9. A mine roof support according to claim 1 wherein said support bearing means is connected to said superstructure of said trailing unit by manually releasable means.
 10. A mine roof support according to claim 1 wherein said hydraulic circuit means includes means for proportioning flow of hydraulic fluid from a pressure source respectively to said double-acting prop or props and to said advancing means to provide an increase in the ratio of lift of said base element to said leasing unit to advancement thereof.
 11. A mine roof support according to claim 1 wherein: a. said double-acting piston and cylinder means are incorporated in at least the foremost props of said outer unit, b. said props and the portion of said base element of said inner unit extending between these props are at all stages of advancement of both of said units contained within the length of said gap afforded by said base element of said outer unit.
 12. A mine roof support according to claim 11 wherein said double-acting piston and cylinder means is incorporated also in at least the foremost prop of said inner unit.
 13. In a mine roof support comprising inner and outer units each having a base element, longitudinally spaced hydraulically extensible props thereon, and roof-engaging superstructure carried by the props and projecting forwardly of the base element of the outer unit, with the base element and superstructure of the inner unit disposed in longitudinal gaps defined between laterally spaced members of the base element and superstructure of the outer unit, and the two units being relatively longitudinally movable by hydraulically energisable advancing means, the improvement comprising: a. support bearing means on said part of the superstructure of the upper unit which projects forwardly of the base element of the outer unit, b. a contact face on that part of the superstructure of the outer unit which projects forwardly of the base element thereof and engageable with said support bearing means upon lowering of the superstructure of said outer unit, c. double-acting piston and cylinder means in at least the foremost props of said outer unit which latter are liftingly connected with the base element of said outer unit.
 14. The improvement claimed in claim 13 wherein said support bearing means comprises: a. shank means supported from said part of the superstructure of said inner unit and projecting downwardly therefrom, b. laterally projecting members carried by said shank means adjacent to its lower end and underlying respective ones of said laterally spaced members of the superstructure of said outer unit.
 15. The improvement claimed in claim 13 wherein said support bearing means further includes rollers on said laterally projecting members beneath respective ones of said laterally spaced members of the superstructure of said outer unit to provide rolling engagement therewith during advancement of said outer unit.
 16. The improvement claimed in claim 13 wherein said support bearing means is urged upwardly with respect to said part of the superstructure of said inner unit by spring means acting between said part and said support bearing means. 